In many industries there is a need to separate fine particulate material from a mixture of coarse particulate material and fine particulate material.
As a particular example, granular polysilicon as produced, e.g., by a fluid bed reactor, such as the reactor shown in U.S. Pat. No. 8,075,692, typically contains from 0.25% to 3% powder or dust by weight. The powder may render the product unsuitable for certain applications. For example, a product containing such levels of powder typically is unsuitable for use in producing monocrystalline silicon because the powder can cause a loss of structure, making single crystal growth impossible.
Current wet processes for removing dust have disadvantages because there is complex, costly equipment to maintain, significant quantities of water and/or chemicals are required, and the processing may cause detrimental oxidation of the polysilicon. Dry processes may avoid these disadvantages, but because silicon powder is highly abrasive, mechanical equipment used in a dry process is subject early failure due to abrasion of the equipment by contact with the silicon materials, particularly at locations where silicon materials enter into spaces between moving parts of the equipment.
Thus there is a need for improved devices and methods for producing granular polysilicon with reduced dust or powder levels.